Added async support

[PhilippTh]

Summary

This PR implements comprehensive asynchronous support for django-constance, making configuration access seamless and efficient in both sync and async Django environments.

Why?

Previously, using Constance in an async context (like a Django async def view) was problematic:

1. Thread Blocking: Accessing config.KEY synchronously inside an async view blocks the event loop, negating the performance benefits of async.
2. Database Errors: For the Database backend, Django's safety guards would raise a SynchronousOnlyOperation error because database queries are prohibited on the main async thread.
3. Manual Overhead: Developers had to manually wrap every Constance access in await sync_to_async(lambda: config.KEY)(), which is verbose and prone to errors.

This implementation removes these barriers by making the config object "async-agnostic"—it detects the execution context and provides the correct, non-blocking path automatically.

How to Use

1. Simple Async Access

In an async view or function, you can now simply await any configuration key:

async def my_view(request):
    # Non-blocking async retrieval
    if await config.MY_SETTING:
        ...

2. Async Updates and Bulk Retrieval

New native async methods have been added for updates and batch operations:

# Update a setting asynchronously
await config.aset('MY_SETTING', 'new_value')
# Fetch multiple settings in one non-blocking call
values = await config.amget(['SETTING_A', 'SETTING_B'])

3. Gradual Migration & Safety Guards

If you forget to use await inside an async loop, the app will still work (to prevent crashes), but it will:

• Emit a Warning: A RuntimeWarning will be triggered: "Synchronous access to Constance setting 'KEY' inside an async loop. Use 'await config.KEY' instead."
• Database Safety: If using the Database backend, it will still raise SynchronousOnlyOperation if accessed synchronously without await, nothing changed here.
• Any previous implementations that use await sync_to_async(lambda: config.KEY)() will continue to work without any issues or drawbacks. Dropping the sync_to_async-lambda combination will increase performance or at the very least developer experience though.

Key Technical Changes

• AsyncValueProxy: A lazy-loading proxy that implements await while supporting standard Python operators (==, +, bool(), etc.) for backward compatibility.
• Native Backend Support:
  • DatabaseBackend now uses afirst() and afetch() for non-blocking I/O.
  • RedisBackend leverages redis.asyncio for native speed.
• Context Detection: Uses asyncio.get_running_loop() to switch behavior at runtime without performance penalty in synchronous code.

Verification

• Added tests/test_async.py covering all new async paths.
• All existing synchronous tests pass.

[codecov]

Codecov Report

❌ Patch coverage is 86.08696% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.20%. Comparing base (d2b8ca1) to head (5a9f416).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
constance/backends/redisd.py	75.94%	12 Missing and 7 partials ⚠️
constance/base.py	93.10%	3 Missing and 3 partials ⚠️
constance/backends/database.py	88.88%	3 Missing and 1 partial ⚠️
constance/backends/__init__.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #656      +/-   ##
==========================================
- Coverage   88.28%   88.20%   -0.08%     
==========================================
  Files          21       21              
  Lines         751      975     +224     
  Branches      123      167      +44     
==========================================
+ Hits          663      860     +197     
- Misses         62       80      +18     
- Partials       26       35       +9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.